Low-load forced air distribution
Hello all. We’re planning a new 3 story home in San Francisco (Climate Zone 3C). I’m thinking of a Chiltrix CX34 for hot water, heat and some limited cooling on the 3rd floor. My Manual J shows 28k in total heating load. We’ll probably do some limited cooling on the 3rd floor with fan coils and/or radiant cooling.
I was initially planning to use zoned hydronic radiant floors everywhere but after reading some of the articles here I’m thinking it may not be worth the cost. For the main (2nd) floor, we don’t want visible minisplit heads/radiators/fan coils/etc so I’m contemplating a ducted fan coil. I also like that the air handler could provide filtration when we have wildfire smoke or other indoor pollutants. The floor is 1291SF and my Manual J shows 9600BTUs of heating load (out of 28k total). The air return would be at the bottom of the stairs on the 1st floor. I’ve attached a drawing showing some potential locations for supply registers based on how difficult it is to duct to them (easy/ok/harder). My main question is: should we go through some additional cost to route a supply duct into the kitchen or will temperature naturally equalize across the open plan? Could we get even temperature distribution with only 1-2 supply registers given the low loads and open plan?
I’m also considering some alternatives that conceal 2+ fancoils:
1. Use a mix of in-floor and kickspace fancoils (e.g. Smith’s FL100 and RS2000 series). I like the simplicity, but I’m a bit worried the fans will be audible. I suspect I’d end up making myself some kind of custom faceplate at least for the FL100s which would be a bit of work.
2. Use a couple fancoils like the Chiltrix CXI series and figure out a way to hide them that I’m happy with (e.g. integrate into the island in the kitchen, some built-in in the living room). I imagine it’d be a good bit of either my own fabrication labor or cost to conceal them.
3. Splurge on a couple Jaga fancoils that recess into the walls/ceilings/floors mostly invisibly. Their units seem to be 1.5-2x the cost of other fancoils (plus the cost of recessing them with an access panel) but they look great!
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I know SF is pretty mild, but what's your heating design temperature? If you look at the chart here, (https://www.chiltrix.com/CX34-air-to-water-heat-pump/ ) the Chiltrix starts losing capacity around 32F and to get 28K BTU at 32F you need to keep your water temperature at 95F or lower. That's kind of marginal for fan units, they're going to be de-rated, which means you need bigger ones, which is kind of the opposite of what you're trying to do.
What is your cooling load like? I've been to SF a couple of times and my recollection is that it's pretty humid in the summer. If you need to be doing dehumidification you need some sort of fan unit for that. My recommendation would be to size your air handlers for your cooling needs, and if you still need heating after that then think about where to put supplemental heat.
With a new house presumably it's going to be quite air-tight and you're going to need mechanical ventilation. You can get systems that monitor the air quality and only vent when it's necessary, otherwise they recirculate. Those are good for evening out the air distribution throughout the house.
Our 99% design temp is 40F, so extremely mild.
Cooling loads are sporadic--the 1% design temp is only 78F. Cooling is a convenience for the heat waves that are becoming more common (but still infrequent). It does feel humid here, but only at the low summer temperatures. The 1% dewpoint is only 59F so dedicated dehumidification is uncommon. We're also on the warmer/dryer side of the city which makes a very large difference in summer humidity (microclimates here are dramatic!).
I'm planning for CO2-driven mechanical ventilation, though that system was going to be entirely separate to keep the controls a little simpler.
Thanks!
If you have decided against whole house in-floor heating, I'd skip the Chilltrix and get a ducted air-to-air heat pump. Much more vanilla so you'll be able to find someone to install the thing. You can add targeted electric resistance in-floor heating for some spaces, like bathrooms, if you want.
It's also easy to separate out DHW from central heating.